1. Field of the Invention
The present invention relates to a sheet processing apparatus and an image forming apparatus, and more particularly to a structure of holding means for holding sheets aligned for processing. The present invention provides a sheet processing apparatus and an image forming apparatus which can stably hold sheets and appropriately align the sheets.
2. Related Background Art
Conventionally, for example, as an image forming apparatus such as a copying machine, a printer, and a facsimile, there is known an image forming apparatus including a sheet processing apparatus which, in order to reduce time and labor required for binding processing and the like for sheets such as copy paper having images formed thereon, sequentially takes the sheets with images formed thereon into the image forming apparatus and selectively applies binding processing, punching processing, and the like to the sheets. As such a sheet processing apparatus, there is known a sheet processing apparatus which temporarily stacks the sheets taken in on an intermediate stacking portion, aligns the sheets, and then performs the binding processing and the like by a stapler to the sheets.
In stacking the sheets on the intermediate stacking portion in this way, an aligned preceding sheet already stacked on the intermediate stacking portion may be pushed out by a subsequent sheet. In this case, alignment of the sheets is not performed appropriately.
Thus, in stacking the sheets on the intermediate stacking portion in this way, in order to prevent the aligned preceding sheet already stacked on the intermediate stacking portion from being pushed out by the subsequent sheet, holding means for holding the preceding sheet after alignment of the preceding sheet is ended is provided.
As such the holding means, for example, there is known a holding means which is arranged outside a sheet conveyance area to prevent the subsequent sheet from colliding with the holding means and holds sheets after sheet conveyance is ended (see Japanese Patent Application Laid-Open No. 2003-73014).
As an image forming apparatus which stacks sheets on an intermediate stacking portion and then switches the sheets back with an intermediate roller to press the sheets against a reference wall for regulating a position of the trailing edges of the sheets, there is known an image forming apparatus which holds sheets using a guide member for guiding the sheets to the reference wall. Note that, when the sheet is held by the guide member in this way, a pressing force of the guide member is weak. Thus, there is known an image forming apparatus which returns a sheet to a reference wall side using belt means formed of a frictional member such as rubber (see Japanese Patent Application Laid-Open No. H09-156820).
However, in such the conventional sheet processing apparatus and image forming apparatus, when the holding means is arranged outside the sheet conveyance area, it is possible to hold a sheet at one end in a width direction of the sheet. Thus, when a push-out force generated by a subsequent sheet is applied to an opposite end of the sheet which is not held, a force for holding the sheets (a holding force) becomes unstable. As a result, a preceding sheet may be pushed out.
When sheets are held by the guide member and returned to the reference wall side by the belt means such that a preceding sheet is not pushed out, if priority is given to a force for returning the sheets (a returning force), a force for gripping the sheets (a gripping force) has to be increased, for example, by increasing a frictional force of a back belt. However, when the gripping force is increased in this way, in aligning the sheets in a width direction thereof, the gripping force of the back belt always in contact with the sheets hinders the alignment. When the gripping force of the back belt is decreased, the holding force becomes unstable. As a result, the preceding sheet may be pushed out.